1. Field of the Invention
The present invention relates to the processing of steel parts, and more specifically to the quenching of parts having undergone thermal steps, especially of carburization, i.e. an introduction of carbon into the surface of the parts to improve their hardness. The present invention more specifically relates to the quenching of parts having undergone a carburizing process under vacuum or under a low gas pressure (lower than atmospheric pressure).
2. Discussion of the Related Art
A low-pressure carburizing process consists of submitting the parts to be processed, in an air-tight chamber, to an alternation of steps of enrichment in the presence of a low-pressure carburizing gas and of diffusion under vacuum or under a low-pressure neutral atmosphere. The respective durations of the enrichment and diffusion steps as well as their number especially depend on the desired carbon concentration and carburizing depth in the part, and such processes are well known in the art. An example of a low-pressure carburizing process is described in French patent application No 2,678,287 of the applicant.
Any carburizing process is followed by at least one quenching step performed either in oil, or in a gas. A main purpose of the quenching is to obtain a fast cooling down of the carburized parts without altering the obtained surface state. A gas quenching is often preferred since it enables directly obtaining dry and clean parts. It is generally desired to obtain the fastest possible cooling rate. To increase the quenching speed with a given gas, the gas mass flow must be increased, that is, the speed and/or the static pressure of the quenching gas must be increased.
Among generally used quenching gases, nitrogen conventionally is an acceptable compromise in terms of cost and efficiency. Nitrogen is indeed often preferred to neutral gases such as helium and hydrogen which, although lighter, and thus easier to convey under a relatively high pressure, are too expensive (helium) or too dangerous (hydrogen).
It would however be desirable to reduce the cost of the quenching step which, due to the gas atmosphere desired to be maintained and to the required mass flow, is not negligible in the general cost of the part processing.
Further, a disadvantage of the use of a gas such as nitrogen or other is, in addition to its cost, the need for conveyance and storage of large volumes. Indeed, industrial gas quenching chambers often have volumes of several cubic meters, or even of several tens of cubic meters.
The quenching process must comply with several constraints, especially relating to the previous carburization. First, the quenching must not alter the surface hardness of the carburized part. Further, the quenching must be fast to satisfy the fast cooling down of the part and not damage its surface. Moreover, an aspect requirement of the obtained part must most often be met, this part not only generally having to have a smooth surface state, but also having to be of the color of steel (grey). In particular, the part must not have a blackened aspect, letting an oxidation be suspected.
The present invention also refers to carbonitriding, having as only difference with carburization that ammonia is generally added to the enrichment gas used. The well known result thereof is the forming of nitrides (instead of carbides for the carburization) at the part surface. It should thus by noted that all that will be discussed hereafter in relation with carburization also applies to carbonitriding.
An object of the present invention is to provide a novel quenching method that overcomes the disadvantages of known methods.
The present invention aims, in particular, at enabling implementation of a particularly economical quenching processing.
Another object of the present invention is to provide a method which is compatible with a conventional low-pressure carburizing process.
Another object of the present invention is to provide a method that respects the surface aspect of the finished parts.
To achieve these objects, the present invention provides a method for quenching steel parts having undergone a low pressure thermal process, which consists of submitting the parts to an air flow at high pressure.
According to an embodiment of the present invention, the air pressure ranges between 5 and 50 bars.
According to an embodiment of the present invention, the quenching duration is shorter than 15 minutes and, preferably, shorter than 2 minutes.
According to an embodiment of the present invention, the parts are not exposed back to air at atmospheric pressure between the low-pressure thermal process and the high-pressure air-quenching.
The present invention also provides a method for processing parts including a low-pressure carburizing process followed by a quenching step.
According to an embodiment of the present invention, the carburizing process includes an alternation of enrichment steps at low pressure in the presence of a carburizing gas and of diffusion steps in the presence of a neutral gas substantially at the same pressure as the enrichment steps.
According to an embodiment of the present invention, the parts are submitted, after the quenching step, to a shot blasting step for, especially, suppressing undesirable surface bumps.
The present invention further provides a thermal processing installation including means for implementing the above processing method.
According to an embodiment of the present invention, the installation includes several processing cells adapted to being tightly isolated from the outside, and handling means for transferring a load from one cell to another, one of these cells forming a quenching cell adapted to being further isolated from the rest of the installation to implement an air-quenching.
According to an embodiment of the present invention, the quenching cell is also used as a cell for unloading the load at the end of the processing.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.